The invention relates to a control mechanism for regulating the fuel-air mixture supplied to an internal combustion engine. The invention relates more particularly to a control mechanism of this type which also includes a sensor for detecting the composition of the exhaust gas and for generating a suitable signal related thereto and a comparator circuit which compares the output signal from the oxygen sensor with a set point or threshold signal. The resultant signal would be supplied to a subsequent mixture preparation system which includes a variety of electronic circuitry for adaptation of the mixture to other requirements. The exhaust gas sensor is an oxygen sensor of the type which provides an output signal that alternates between a high and a low voltage depending on whether the exhaust gas contains excess oxygen or not, i.e., depending on whether the fuel-air mixture is lean or rich. When the oxygen sensor is not ready for operation, for example when it is not yet at its normal operating temperature, the signal which it generates cannot readily be used or provisions must be made to adapt the system to processing an abnormal signal. Known in the art is a device for use within a sensor-control fuel injection system which completely turns off the control process when the .lambda.-sensor is found to be input non-operational and which switches the system over to forward open-loop control. Further circuits then adapt the threshold voltage with the increasing .lambda.-output signal after the control process is reengaged until the sensor is found to be operating completely normally. In this known apparatus, very complicated circuitry is required to achieve the desired result.